Route guidance device

ABSTRACT

Frequent execution of auto-rerouting in the vicinity of a junction where a current-position skipping of a vehicle due to map-matching is likely to occur is prevented. 
     When the current position of the vehicle calculated by a current position calculating unit  11  is in the vicinity of a junction on a recommended route set by the route searching unit  13  and even if the current position of the vehicle is off the recommended route as a result of correction by a map matching unit  12,  a route guidance unit  132  does not request a route searching unit  131  to search a newly recommended route extending from the corrected current position of the vehicle to a destination.

TECHNICAL FIELD

The present invention relates to a technology for calculating arecommended route from a starting point to a destination, and suggestingit to a user in an on-board navigation device or the like.

BACKGROUND OF THE INVENTION

In an on-board navigation device, a technology (a route guidance) hasbeen already put into a practical use, which searches a recommendedroute from a set starting point to a set destination based on road mapdata, and guides a vehicle to reach the destination on the recommendedroute by using a display device, a voice output device and the like.

In the route guidance technology, all routes connecting the set staringpoint with the set destination are searched by using road map data, anda technology called a dijkstra method searches for a route of minimumcost (for example, the gross distance of the route), thereby setting thesearched route as the recommended route. And, for example, the setrecommended route is displayed while overlapping a displayed map, andinformation to be required (for example, information such as whether ornot to turn the next intersection) for a vehicle to run on therecommended route is informed via voice. In a case when the vehicledeviates from the recommended route during the route guidance, arecommended route to the destination is searched again, taking thecurrent position of the vehicle as a starting point, and the re-searchedroute is newly set as the recommended route (an auto-rerouting).

In addition, in an on-board navigation device, in order to position avehicle on a road in a displayed map, a technology (a map matching) hasalready been put into a practical use, wherein the current position ofthe vehicle, determined by various kinds of sensors such as gyro, arunning vehicle speed sensor and the like, or GPS receiver, is correctedevery specified running distance (for example, 20 meters).

In this map matching technology, the current position of the vehicle,determined by various kinds of sensors such as the running vehicle speedsensor, a gyro or the like, and a GPS receiver, is regarded as a center.Then, all positions corresponding to the current position on the roadwithin a specified range are registered as candidate positions, thespecified range being set by the searched current vehicle position,errors included in a traveling direction, and errors of road map data.Then, a current vehicle position is corrected to a candidate positionwhich has the strongest correlation with the current position, inconsideration for a distance from a current position to the candidateposition; a difference between a vehicle traveling direction, searchedby various kinds of sensors such as the vehicle speed sensor, the gyroor the like and the GPS receiver or the like, and a azimuth of road inwhich a candidate position is set; and a road having the currentposition of the vehicle corrected by the previous map matching.

DISCLOSURE OF THE INVENTION

Incidentally, in the on-board navigation apparatus to which theforegoing map matching technology is applied, in the case where theplurality of candidate positions are registered, the current positioncorrected by the map matching at this time is set on a road differentfrom the road with the current position corrected by the previous mapmatching being set therein (a so-called position skipping). Hereinbelow,a description will be made for this position skipping phenomenon withreference to drawings.

FIG. 5 is a view for explaining the position skipping phenomenon in theon-board navigation apparatus to which the map matching technology isapplied. Here, FIG. 5(a) shows an actual road, and FIG. 5(b) shows ashape of a road determined by the road map data corresponding to theroad shown in FIG. 5(a).

Now, in FIG. 5(a), it is assumed that the vehicle changes a lane beforea junction A and goes straight, then the map matching is performedimmediately after the vehicle passes the junction A. In this case,according to the foregoing map matching technology, a position B on amain lane and a position C on a branch lane, which are shown in FIG.5(b), are registered as candidate positions, both having strong anduncomparable correlations with the current position of the vehicledetermined by various kinds of sensors such as a vehicle speed sensorand a gyro, or a GPS receiver and the like. However, when a differencebetween a traveling direction of the vehicle, searched by the variouskinds of sensors such as the vehicle speed sensor and the gyro, or theGPS receiver and the like, and an azimuth of each road in which thecandidate position is set is considered, there is a strong possibilitythat not the candidate position on the branch lane where the vehicleactually runs but the candidate position B on the main lane is selectedas a current position after correction.

Here, it is assumed that the current position is corrected to thecandidate position B by the map matching, further the vehicle runsthereon. Then, it is assumed that map matching is performed when thevehicle reaches a point D on the branch lane. Also in this case, aposition E on the main lane and a position F on the branch lane, whichare shown in FIG. 5(b), are registered as candidate positions. However,in consideration for a distance from the current position of the vehicleto the candidate position thereof, which is determined by the variouskinds of sensors such as the vehicle speed sensor and the gyro or theGPS receiver and the like; or for a difference between the travelingdirection of the vehicle, determined similarly to the above by thevarious kinds of sensors such as the vehicle speed sensor and the gyroor the GPS receiver and the like, and an azimuth of each road where thecandidate position is set, there is a strong possibility that thecandidate position F on the branch lane is selected as a currentposition after correction.

Here, if it is assumed that the current position is corrected to thecandidate position F by the map matching, the current position beingdisplayed on the display device is suddenly skipped from the main laneonto the branch lane at the point D. This position skipping phenomenonhas a possibility of continuing until a relative distance between themain lane and the branch lane or a difference in shapes between the mainlane and the branch lane are made apparent.

Now, when the route guidance technology equipped with theabove-described auto-rerouting function is further applied to theon-board navigation apparatus to which the map matching technologyinvolving the position skipping phenomenon as described above isapplied, a problem to be described below occurs.

Now, in FIGS. 5(a) and 5(b), it is assumed that a route connecting themain lane through the junction A to the branch lane is selected as arecommended route, and that the route guidance is made so that thevehicle may run on the route. Here, as described above, in the casewhere the vehicle changes the lane before the junction A and goesstraight to pass the junction A, leading to the correction of thecurrent position to the candidate position B on the main lane by the mapmatching, it results in that the vehicle deviates from the recommendedroute during the route guidance, thus the auto-rerouting is executed.Specifically, the corrected current position B is set as starting point,a recommended route to the destination is searched again, and there-searched route is newly set as a recommended route. Thus, the routeguidance is performed.

Moreover, as described above, in the case where the map matching isperformed when the vehicle reaches the point D on the branch lane, andas a result, the current position is corrected to the candidate positionF on the branch lane, it results in that the vehicle deviates from therecommended route searched again by the previous auto-rerouting.Accordingly, the auto-rerouting is executed again. Specifically, thecorrected current position F is set as a starting point, a recommendedroute to the destination is searched again, and the re-searched route isnewly set as a recommended route. Thus, the route guidance is performed.

As described above, when the route guidance technology equipped with theauto-rerouting function is applied to the on-board navigation apparatusto which the map matching technology is applied, the auto-rerouting isexecuted every time when the current position is subjected to theposition skipping, and a different recommended route is submitted to theuser, which is awfully troublesome.

The present invention was made with the foregoing circumstance in mind.An object of the present invention is to prevent the execution of theauto-rerouting every time when the position skipping is made in a routeguidance device such as the on-board navigation apparatus to which themap matching technology is applied.

In order to solve the foregoing subject, the present invention is aroute guidance device, in which a recommended route from a set startingpoint to a set destination is searched to be submitted to a user; andwhen a current position of a vehicle deviates from the recommendedroute, a recommended route from the current position of the vehicle tothe destination is newly searched to be submitted to the user, the routeguidance device comprising:

data storing means for storing road map data;

current position measuring means for measuring a current position of avehicle;

map matching means for correcting the current position of the vehiclemeasured, for each specified interval, by the current position measuringmeans so as to be located on any road in a specified range from thecurrent position among roads determined by the road map data stored inthe data storing means; and

auto-rerouting preventing means for preventing a search for a newrecommended route from a corrected current position of the vehicle tothe destination even if the current position of the vehicle is correctedby the map matching means and deviates from the recommended routeresultantly when the current position of the vehicle measured by thecurrent position measuring means is in the vicinity of a junction on therecommended route among the roads determined by the road map data storedin the data storing means.

Here, a judgment as to whether or not the current position of thevehicle measured by the current position measuring means is in thevicinity of the junction on the recommended route among the roadsdetermined by the road map data stored in the data storing means may beperformed, for example, in the following manner. Specifically,information regarding the junctions is previously set in the road mapdata stored in the data storing means, and among the junctions stored inthe data storing means, the junction on the recommended route, which isproximate to the current position of the vehicle measured by the currentposition measuring means, is searched. Then, investigation is made as towhether or not the current position of the vehicle exists within aspecified distance from the junction in the traveling direction of thevehicle (for example, the distance is within 1000 m in the case wherethe recommended route is a highway, and is within 400 m in the casewhere the recommended route is an open road).

Alternatively, the judgment may be made by investigating whether or notthe vehicle moves for a specified distance after the number of roadsselected to be within a specified range from the current position of thevehicle measured by the current position measuring means is increased inthe map matching. means. In this case, when a value determined by therunning distance of the vehicle after it is judged that the currentposition of the vehicle is in the vicinity of the junction on therecommended route and the azimuth difference among a plurality of roadsselected to be within the specified range from the current position ofthe vehicle measured by the current position measuring means in the mapmatching means exceeds a predetermined reference value, the currentposition of the vehicle is corrected by the map matching means, and as aresult, the prevention of the search for a new recommended route fromthe corrected current position of the vehicle to the destination in thecase where the vehicle deviates from the recommended route may bereleased.

According to the present invention, by the foregoing configuration, inthe case where the vehicle is located in the vicinity of the junctionwhere there is a strong possibility that the position skipping occursdue to the map matching, the search for the new recommended route fromthe current position of the vehicle to the destination is prevented evenin the case where the vehicle deviates from the recommended route.Therefore, a situation such that the auto-rerouting is executed everytime when the current position is subjected to the position skipping,and a different recommended route is submitted to the user can beprevented from occurring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic constitutional view of a navigation device towhich a first embodiment of the present invention is applied.

FIG. 2 is a flow chart for explaining an operation of the navigationdevice to which the first embodiment of the present invention isapplied.

FIG. 3 is a view for explaining a principle of an azimuth differenceevaluation value L used in a second embodiment of the present invention.

FIG. 4 is a flow chart for explaining an operation of the navigationdevice to which the second embodiment of the present invention isapplied.

FIG. 5 is a view for explaining a position skipping phenomenon in anon-board navigation to which a map matching technology is applied.

BEST MODE FOR PERFORMING THE PRESENT INVENTION

Hereafter, a description will be made with an example in which a firstembodiment of the present invention is applied to an on-board navigationdevice.

FIG. 1 is a schematic constitutional view of a navigation device towhich a first embodiment of the present invention is applied.

As shown in the drawing, a navigation device of the present embodimentcomprises a processor device 1 consisting of a micro processor and amemory; a display device 2 such as a liquid crystal display; a drivedevice 3 for accessing a storage medium 8 such as a CD-ROM with map datarecorded therein and for reading data; an input device 4 for receivinginstructions from users; a vehicle speed sensor 5 for detecting avehicle speed by the revolving number of a tire per unit time; a GPSreceiver 6 for making GPS data and sequentially outputting the data; anazimuth sensor 7 such as a geomagnetic sensor and a gyro for measuringan azimuth of a vehicle; and a voice output device 9. The GPS receiver,specifically, receives signals issued from at least three GPSsatellites; measures a distance to each satellite and change rates ofthe distances every specified time (for example, one second); makes dataincluding a current position of a vehicle, a speed, and a travelingdirection (an azimuth) by the obtained result; and outputs the datasequentially.

In this embodiment, the display device 2, the drive device 3, the inputdevice 4, the vehicle speed sensor 5, the GPS receiver 6, the azimuthsensor 7, and the voice output device 9 are the same as those havingbeen conventionally used for navigation devices.

The processor device 1 realizes a current position calculating unit 11,a map matching unit 12, and a navigation unit 13 as a process bymicro-processor which runs a specified program on a memory. Note that,the specified program may be previously stored in the memory, or may beloaded in the memory from the storage medium 8 such as a CD-ROM via thedrive device 3.

The current position calculating unit 11 sequentially calculates acurrent position of a vehicle and a traveling direction thereof based onGPS data sequentially outputted from the GPS receiver 6 every specifiedtime (for example, one second), and data measured by the various kindsof sensors 5 and 7. These processes in the current position calculatingunit 11 are the same as the current position calculating processes ofthe vehicle performed in the conventional navigation device.

The map matching unit 12, regarding the current vehicle positionsearched at the current position calculating unit 11 as a center, everyspecified distance (for example 20 m), takes out information from themap data read out by the navigation unit 13 (to be described later), theinformation being relative to all roads within a specified rangedetermined by errors included in the current position or the travelingdirection of the vehicle and errors of the map data. Subsequently, aposition corresponding to the current position on each road, which wastaken out, is registered as a candidate position. Then, in considerationfor a distance from the current position to each candidate position; adifference between the vehicle traveling direction, searched at thecurrent position calculating unit 11, and a azimuth of a road in whicheach candidate position is set; and a road having the current positionof the vehicle which is corrected by the previous map matching, thecurrent position of the vehicle searched by the current positioncalculating unit 11 is corrected to a candidate position which has thestrongest correlation with the current position. These processes in themap matching unit 12 are the same as the map matching processesperformed in the conventional navigation device.

The navigation unit 13, from the storage medium 8 via the drive device3, reads map data in the vicinity of the current position of the vehicleobtained by the current position calculating unit 11 or the map matchingunit 12, and displays a map according to the data on the display device2. In addition, the navigation unit 13 also displays a mark whichexpresses the current position and the traveling direction of thevehicle searched by the current position calculating unit 11 or the mapmatching unit 12 on the displayed map while overlapping the markthereon.

Moreover, the navigation unit 13 comprises a route searching unit 131and a route guidance unit 132 as a processor device for a routeguidance.

The route searching unit 131 searches, by using the map data stored inthe storage medium 8, all routes connecting a starting point with adestination, which are received from an operator via the input device 4.Thereafter, for example, by a technology called a dijkstra method, aroute of minimum cost (for example, the gross distance of the route) issearched and set as a recommended route (a route searching). Inaddition, when the vehicle deviates from the recommended route duringthe route guidance by the route guidance unit 132 (to be describedlater), a recommended route to a destination is searched again, takingthe current position of the vehicle obtained by the current positioncalculating unit 11 or the map matching unit 12 as a new starting point.Then, the re-searched route is newly set as a recommended route (anauto-rerouting). The route searching and the auto-rerouting processes,described above, are the same as route searching and auto-reroutingprocesses performed in the conventional navigation device.

The route guidance unit 132, by using the display device 2 and the voiceoutput device 9, guides the vehicle to reach the destination on therecommended route set by the route searching unit 131. For example, theroute guidance unit 132 displays the recommended route set by the routesearching unit 131 on the displayed map while overlapping therecommended route thereon, and informs the information required for thevehicle to run on the recommended route (for example, information suchas whether or not to turn the next intersection) via voice (a routeguidance).

Moreover, when the current position of the vehicle obtained by the mapmatching unit 12 deviates form the recommended route set by the routesearching unit 131, the route guidance unit 132 instructs the routesearching unit 131 for auto-rerouting, obtains a new recommended routefrom the route searching unit 131, and performs the foregoing routeguidance according to the route. However, when the current position ofthe vehicle searched by the current position calculating unit 11 is inthe vicinity of a junction on the recommended route, the route guidanceunit 132 does not instruct the route searching unit 131 forauto-rerouting as a result of a correction of the current position ofthe vehicle by the map matching unit 12, even if the current position ofthe vehicle deviates from the recommended route. By performing this, anauto-rerouting in the vicinity of the junction on the recommended routeis prohibited.

Herein, a judgement of whether or not the current position of thevehicle searched by the current position calculating unit 11 approachesin the vicinity of the junction on the recommended route will beperformed as below.

Specifically, when the number of candidate positions registered in themap matching unit 12 is increased, since it indicates that the number ofroads within a specified range is increased more than the number of thecurrent position of the vehicle searched by the current positioncalculating unit 11, the possibility of the vehicle approaching thejunction is high. Thus, when the number of candidate positionsregistered in the map matching unit 12 is increased, it is judged thatthe current position of the vehicle approaches in the vicinity of. thejunction on the recommended route.

On the other hand, a judgement of whether or not the current position ofthe vehicle searched by the current position calculating unit 11 is offthe vicinity of the junction on the recommended route may be determinedby, for example, searching a running distance (for example, a movingdistance of the current position of the vehicle which is sequentiallycalculated by the current position calculating unit 11) after it isjudged that the current position of the vehicle obtained by the currentposition calculating unit 11 approaches in the vicinity of the junctionon the recommended route. Specifically, in the case where therecommended route is set on a high way, when the vehicle runs 1000meters, alternatively, in the case where the recommended route is set onthe open road, when the vehicle runs 400 meters, it may be judged thatthe current position of the vehicle is off the vicinity of the junctionon the recommended route.

Next, a description will be made for an operation of the navigationdevice, to which the first embodiment of the present invention isapplied, with the above-described constitution.

FIG. 2 is a flow chart for explaining an operation of the navigationdevice to which the first embodiment of the present invention isapplied. This flow is started, for example, when a starting point and adestination are set by a user via the input device 4, and a recommendedroute connecting the starting point with the destination is searched andset. Then, the flow is repeatedly executed until a vehicle reaches thedestination, or a cancel of a route guidance is instructed by a user viathe input device 4.

First, the current position calculating unit 11 calculates a currentposition and a traveling direction of a vehicle by using data outputtedfrom the GPS receiver 6 and the various kinds of sensors 5 and 7 (stepS101).

Next, the map matching unit 12 judges whether it is time for performinga map matching to the current position calculated by the currentposition calculating unit 11 (step S102). For example, if the mapmatching is performed every 20 meters of running, the map matching unit12 judges whether or not the vehicle has run 20 meters since the lastmap matching was performed.

Herein, when it is not yet the time for map match timing, the processingadvances directly to step Sill, and the route guidance unit 132 performsthe route guidance described above by using the current position and thetraveling direction of the vehicle calculated in step S101 and therecommended route set by the route searching unit 131. Thereafter, theprocessing returns to step S101.

On the other hand, when it comes the time for map match timing in stepS102, the processing advances to step S103, and the map matching unit 12performs the above-described map matching process on the currentposition of the vehicle calculated in step S101. Thereafter, theprocessing advances to step S104.

In step S104, the route guidance unit 132 judges whether or not thenumber of candidate positions of the map matching process, performed instep S103, is increased and larger than the number of candidatepositions of the map matching process previously performed. As describedabove, when the number of candidate positions registered in the mapmatching unit 12 is increased, it indicates that the number of roadswithin a specified range is increased more than the number of thecurrent position of the vehicle searched by the current positioncalculating unit 11. Therefore, in this case, the possibility of thevehicle approaching in the vicinity of a junction is high. Then, whenthe number of candidate positions is increased, the vehicle is judged ashaving approached in the junction, the processing advances to step S105,and a flag for prohibiting auto-rerouting turns ON. Thereafter, theprocessing advances to step S106. On the other hand, when the number ofcandidate positions is not increased, the processing advances directlyto step S106 without executing step S105.

In the step S106, the route guidance unit 132 judges whether or not theflag for prohibiting auto-rerouting is turned ON. When the flag is ON,specifically, when auto-rerouting is prohibited, the processing advancesto step S107, and it is judged whether or not the vehicle has run aspecified distance (for example, 1000 meters on a high way, and 400meters on an open road) since the flag was turned ON. When the vehiclehas run the specified distance, it is judged that the vehicle is off thevicinity of the junction, the flag is turned OFF to cancel theprohibition of auto-rerouting (step S108), and the processing advancesto step S109. On the other hand, when the vehicle has not run thespecified distance, the processing advances directly to step S111, andthe above-described route guidance is performed by using the setrecommended route and the traveling direction of the vehicle and therecommended route corrected by the map matching process. Thereafter, theprocessing returns to step S101.

In the step S106, when the flag is OFF, specifically, whenauto-rerouting is not prohibited, the processing advances to step S109without performing the above-described steps S107 and S108, and it isjudged whether or not the current position of the vehicle corrected bythe matching process in step S103 is on the recommended route. Then,when it is on the recommended route, the processing advances to stepS111, and the above-described route guidance is performed by using therecommended route, the traveling direction and current position of thevehicle corrected by the matching process. Thereafter, the processingreturns to step S101. On the other hand, when the current position ofthe vehicle is not on the recommended route, the above-describedauto-rerouting is performed by setting the current position of thevehicle corrected by the matching process as a starting point, and a newrecommended route is set. Thereafter, the processing advances to stepS111, and the above-described route guidance is performed by using thenewly set recommended route, the traveling direction of the vehicle, andthe current position corrected by the matching process. Then, theprocessing returns to step S101.

Hereinabove, the description has been made for the first embodiment ofthe present invention.

In this embodiment, as described above, when the vehicle running on therecommended route approaches the vicinity of the junction,auto-rerouting is prohibited until the vehicle runs the specifieddistance. By conforming to the foregoing, a situation can be preventedfrom occurring, in which auto-rerouting is executed for every positionskipping of the current position of the vehicle in the map matching inthe vicinity of a junction where a position skipping of the currentposition of the vehicle is likely to occur, and a different recommendedroute is suggested to a user.

Note that, in the above-described embodiment, a judgement whether or notthe vehicle running on the recommended route approaches in the vicinityof a junction, is made based on whether or not the number of candidatepositions in the map matching process increases. However, the judgementmay be performed, for example, by previously setting informationconcerning junctions on road map data stored in the storage medium 8,obtaining the information of the junctions on the recommended route fromthe storage medium 8 by the route guidance unit 132, and judging whetheror not the current position of the vehicle calculated by the currentposition calculating unit 11 reaches any junction whose information isobtained.

Next, a description will be made for a second embodiment of the presentinvention.

In the above-described first embodiment, it is made a condition forcanceling the prohibition of auto-rerouting that the vehicle runs aspecified distance (for example, 1000 meters on a high way, and 400meters on an open road) after auto-rerouting is prohibited. This isbecause it is considered that, in the vicinity of a junction, unless thevehicle runs a certain distance, the superiority and inferiority of acorrelation between each candidate position registered by the mapmatching unit 12 and the current position of the vehicle calculated bythe current position calculating unit 11 is not clear.

However, even if the vehicle does not run the specified distance, thesuperiority and inferiority of the correlation between each candidateposition registered in the map matching unit 12 and the current positionof the vehicle calculated by the current position calculating unit 11 isclear in some cases depending on the road configuration in the vicinityof the junction.

For example, in FIG. 3, as shown in (a), when an angle made by a mainline with a branch line at a junction is narrow and the both lines areapproximately parallel to each other (when an azimuth difference of aroad is small), if a vehicle does not run a certain distance, thesuperiority and inferiority of the correlation between each candidateposition registered in the map matching unit 12 and a current positionof the vehicle calculated by the current position calculating unit 11 isnot clear. However, as shown in (b), when an angle made by a main linewith a branch line becomes gradually large (when an azimuth differenceof a road is large), even if a vehicle does not run a certain distance,the superiority and inferiority of the correlation between eachcandidate position registered in the map matching unit 12 and a currentposition of the vehicle calculated by the current position calculatingunit 11 is clear.

Then, in this embodiment, an evaluation value (an azimuth differenceevaluation value) is introduced, whose value decreases (or increases) asthe superiority and inferiority of the correlation between eachcandidate position registered in the map matching unit 12 and a currentposition of the vehicle calculated by the current position calculatingunit 11 is clear. When the evaluation value becomes smaller (or theevaluation value becomes larger) than a specified threshold, even if thevehicle does not run a specified distance after auto-rerouting isprohibited, the prohibition of the auto-rerouting is canceled.

A schematic construction of the navigation device, to which the secondembodiment of the present invention is applied, is identical to aschematic construction of the navigation device to which the firstembodiment of the present invention shown in FIG. 1 is applied. However,in this embodiment, an azimuth difference evaluation value L isintroduced, whose value decreases as the superiority and inferiority ofthe correlation between each candidate position registered in the mapmatching unit 12 and a current position of the vehicle calculated by thecurrent position calculating unit 11 is clear, and the azimuthdifference evaluation value is calculated every time when the mapmatching is performed by the map matching unit 12 from the time when aflag indicating the prohibition of the auto-rerouting is turned ON tothe time when the flag is turned OFF in the route guidance unit 132. Andwhen the azimuth difference evaluation value is zero or less, even ifthe vehicle does not run a specified distance, the flag is turned OFF tocancel the prohibition of the auto-rerouting.

Herein, a calculating formula of the azimuth difference evaluation valueL will be illustrated.

L=T−(dcar×10+Σθ×G)

Note that in the above-described formula, T is a specified initial valuewhich is set to 3000 by the inventors of the present invention, and G isan arbitrary factor which is set to 11/10 by the inventors of thepresent invention. Dcar is a running distance (m) after the flag isturned ON. Since the azimuth difference evaluation value L is calculatedevery time when the map matching is performed, the azimuth differenceevaluation value L can be obtained by checking how many times the mapmatching is performed after the flag is turned ON. For example, when themap matching is performed every 20 meters of running, and assuming thatthe number of map matchings performed after the flag is turned ON is N,dcar=20 N is obtained. a is a difference (a road azimuth difference)between a road azimuth at a current position of a vehicle on a road,which has the current position of a vehicle corrected by the mapmatching, and a road azimuth at a corresponding candidate position of aroad, which has each candidate position registered by the map matchingand is other than the road having the corrected current position of thevehicle. Information concerning the road azimuth can be obtained fromthe map data stored in the storage medium 8. Note that, when the numberof candidate positions registered by the map matching is three or more,the road azimuth difference θ becomes two or more. Accordingly, theazimuth difference evaluation value L also becomes two or more. In thiscase, only if all azimuth difference evaluation values L are zero orless, the flag is turned OFF and a prohibition of the auto-rerouting iscanceled.

Next, a description will be made for an operation of the navigationdevice to which the second embodiment of the above-described structureof the present invention is applied.

FIG. 4 is a flow chart for explaining an operation of the navigationdevice, to which the second embodiment of the present invention isapplied. This operation flow differs from that of the first embodimentshown in FIG. 2 in that step S201 and 202 are added. With respect toother steps, this operation flow is identical to that of the firstembodiment shown in FIG. 2.

Specifically, in this embodiment, when it is judged that a flag forprohibiting auto-rerouting is turned ON by a route guidance unit 132 instep S106, the processing advances to step S201, and the route guidanceunit 132 calculates the above-described azimuth difference evaluationvalue L. Then, the processing advances to step S202. In step S202, theroute guidance unit 132 judges whether or not the calculated azimuthdifference evaluation value L is zero or less. When the value is zero orless, the processing advances to step S108 without performing step S107,and the flag is turned OFF. On the other hand, when the calculatedazimuth difference evaluation value is more than zero, the processingadvances to step S107.

In the foregoing, the description has been made for the secondembodiment of the present invention.

In this embodiment, as described above, an azimuth difference evaluationvalue is introduced whose value decreases as the superiority andinferiority of the correlation between each candidate positionregistered in the map matching unit 12 and the current position of thevehicle calculated by the current position calculating unit 11 is clear.When the evaluation value is zero or less, even if a vehicle does notrun a specified distance after the flag is turned ON, the flag is turnedOFF. Therefore, a situation can be more efficiently prevented fromoccurring, in which auto-rerouting is executed for every positionskipping of the current position of the vehicle in the map matching inthe vicinity of a junction where a current-position skipping of avehicle is likely to occur, and then a different recommended route issuggested to a user.

As described above, according to the present invention, it becomespossible to prevent the frequent execution of the auto-rerouting in thevicinity of a junction where a current-position skipping of a vehicledue to map matching is likely to occur.

What is claimed is:
 1. A route guidance device, in which a recommendedroute from a set starting point to a set destination is searched to besubmitted to a user; and, when a current position of a vehicle deviatesfrom said recommended route, a recommended route from the currentposition of the vehicle to the destination is newly searched to besubmitted to the user, said route guidance device comprising: datastoring means for storing road map data; current position measuringmeans for measuring a current position of a vehicle; map matching meansfor correcting the current position of the vehicle measured, for eachspecified interval, by said current position measuring means so as to belocated on any road in a specified range from the current position amongroads determined by the road map data stored in said data storing means;and auto-rerouting preventing means for preventing a search for a newrecommended route from the corrected current position of the vehicle tothe destination even if the current position of the vehicle is correctedby said map matching means and deviates from said recommended routeresultantly when the current position of the vehicle measured by saidcurrent position measuring means is in the vicinity of a junction onsaid recommended route among the roads determined by the road map datastored in said data storing means.
 2. The route guidance deviceaccording to clam 1, wherein said auto-rerouting preventing meanssearches the junction on said recommended route, said junction beingnear the current position of the vehicle measured by said currentposition measuring means, among junctions of the roads determined by theroad map data stored in said data storing means, and judges that thecurrent position of the vehicle is in the vicinity of the junction onsaid recommended route among the roads determined by the road map datastored in said data storing means when the current position of thevehicle is within a specified distance from the junction in a travelingdirection of the vehicle.
 3. The route guidance device according toclaim 1, wherein said auto-rerouting preventing means judges that thecurrent position of the vehicle is in the vicinity of the junction onsaid recommended route among the roads determined by the road map datastored in said data storing means from the time when the number of roadsselected as to be within a specified range from the current position ofthe vehicle measured by said current position measuring means isincreased to the time when the vehicle moves a specified distance insaid map matching means.
 4. The route guidance device according to claim3, wherein said auto-rerouting preventing means releases prevention ofthe search for a new recommended route from the corrected currentposition of the vehicle to the destination in the case where the currentposition of the vehicle is corrected, even when the vehicle has notmoved said specified distance, by said map matching means and deviatesfrom said recommended route resultantly when a value exceeds apredetermined reference value in said map matching means, said valuebeing determined by a running distance of the vehicle from the time whensaid current position of the vehicle is in the vicinity of the junctionon said recommended route and by an azimuth difference between aplurality of the roads selected as to be within the specified range fromthe current position of the vehicle measured by said current positionmeasuring means.